Method and system for controlling a tandem rolling mill

ABSTRACT

An automatic gage control system for a tandem rolling mill wherein separate control loops are provided for the roll opening controls of each stand. Each control loop responds to a feedback signal based on the predicted gage of the rolled strip leaving the associated stand whereby control errors caused by transport lag are eliminated without causing system instability.

United States Patent Winchester Tan. 1, 1.974

3,688,532 9/1972 Silva 72/16 Primary Examiner-Milton S. Mehr [75]Inventor: Amos .1. Winchester, Scotia, N.Y. Attmey ]ohn J. Kissane et aL[73] Assignee: General Electric Company, Salem,

22 F1 1 l 1 May 3 N72 57 ABSTRACT [21] Appl. No.: 249,955

An automatic gage control system for a tandem rolling [52] US. Cl. 72/9,72/12 mill wherein separate control loops are provided for 1] Int. ClB211) 37/02 the ll p ning n r l f h n E h control [58] Field of Search72/8-12, loop responds to a feedback signal based on the pre- 72/16dicted gage of the rolled strip leaving the associated stand wherebycontrol errors caused by transport lag [56] References Cited areeliminated without causing system instability.

UNITED STATES PATENTS 3.387.471 6/1968 Freedman .4 72/9 6 Claims, 1Drawing Figure 38 [39 /4O GAGE (49 A 1 :COMPATOR g] SET COMPARATOR 62blb F20 (5 G. 20 50 19 SCREW 44 c; SCREW 4| CONTROL G l 4 g CONTROLCALCULATE B F ER CALCULATE l STORAGE i G (E -G e- I 350 36 45 2b 20 V335 l8 l6 Q 18 47 33 36 46 v, 21 (5V; 2) 52 v 12791;] 13 G '3 u I 1 G ba? I zbm" 1 48 W) l *1 1 l" 1 U I X-RAY '5 l XRAY GAGE 27b 14 i 23 14GAGE BUFFER SPEED TENSION SPEED STORAGE 21/ REG. REG. REG. MONITOR 53BACKGROUND OF THE INVENTION The present invention relates to a methodand system for controlling a tandem rolling mill for reducing, insuccessive rolling operations, the thickness of strip material passingthrough the mill. More particularly, the invention relates to a methodand system for minimizing off gage strip caused by time delay, known astransport lag, in the feedback circuits controlling the automatic gagcontrols of the mill.

Tandem rolling mills have rolling stands provided with screwdownmechansims to vary the roll opening and hence exit gage of rolled strippassing through the mills. The screwdowns are controlled by an automaticgage control system, hereinafter referred to as the AGC system, whichsenses the gage of the strip passing through the mill and adjusts thescrewdown to correct for any departure of the rolled strip gage from adesired value. Typically, the strip thickness is measured by a gage,such as an X-ray gage, which produces a correction feedback signal forthe AGC system when the exit gage departs from a desired value. In asystem of this type, any error in the thickness of the rolled strip isnot sensed unit] a measured section of the strip passes beyond the rollbite of a stand to the measuring gage. This measuring delay in thefeedback control of the AGC system is known as transport lag. The effectof the transport lag is to cause instability or hunting of the controlsystem if the speed of response of the AGC control system is not reducedto compensate for the transport lag or delay time. In actual practicethe response time of the AGC system is normally maintained at leastthree times the transport lag. It is apparent that reduction of responsetime of the AGC system to prevent instability caused by transport lagincreases the amount of off gage strip produced by the mill by slowingdown the correction process. Hence, there is more scrap loss due to offgage material. It is desirable, therefore, to eliminate the effect oftransport lag in the AGC control system of the mill.

Attempts have been made to reduce the effect of transport lag by usingas a feedback signal a predicted gage of the strip leaving the mill. Thepredicted gage is calculated by use of the constant volume principle;i.e. the volume flow rate of the strip into the mill equals the volumeflow rate of the strip out of the mill. Using this principle the exitgage can be calculated by multiplying a measured entry gage by the ratioof the strip speeds entering and leaving the mill. In prior AGC controlsystems using this predicted exit gage as a feedback sigal thescrewdowns of adjacent rolling stands of the mill have been coupledtogether and adjusted simultaneously in response to the feedback signal.This approach gives rise to complex control problems arising from thecoupling together of the adjacent stands. Specifically, it becomesdifficult to maintain the interstand tension of the rolled strip whenthe stands are so coupled and without proper tension regulation thestrip is apt to loop or break. Also, the constant volume principle usedas a basis for predicting exit gage of the strip is valid for the entiremill, as distinguished from individual rolling stands, only when uniforminterstand strip tension is maintained. j

Accordingly, it is an object of the present invention to provide animproved method for controlling the AGC system of a tandem rolling millto eliminate the effect of transport lag.

Another object of the invention is to provide an improved system forcontrolling the rolling stands of a tandem rolling mill to maintain adesired strip exit gage which eliminates the effect of transport lagwithout interconnecting the adjacent rolling stands in such a manner asto cause problems in maintaining or controlling interstand striptension.

Further objects and advantages of the invention will become apparent asthe following description proceeds.

SUMMARY Briefly, in accordance with the invention, AGC control systemfor the individual rolling stands of a tandem rolling mill are providedwith separate control loops. The control loop of each stand receives, asa feedback signal, a predicted exit gage signal for that particularstand. In this manner, the effect of transport lag is eliminated whileavoiding undesired direct intercoupling of the rolling stands. Thepredicted exit gage signal of the first or upstream stand is calculatedby multiplying a measured entry gage signal by the ratio of the stripspeeds entering and leaving the first stand. The predicted exit gagesignal for the second or downstream stand is calculated by multiplyingan entry gage signal by the ratio of the strip speeds entering andleaving the second stand. In this case, the entry gage signal is derivedfrom the exit gage signal of the first stand and is delayed in a bufferstorage unit by the amount of time required for measured strip incrementto pass between the roll bites of the upstream and downstream stands.The calculation of the predicted strip exit gages from the stands ismade by application of the constant volume flow rate principle appliedto the individual stands as distinguished from the mill as a whole. Tocorrect for any small gage errors that might result from improperoperation of the predicted gage control system, a monitor system isprovided which measures strip exit gage leaving the mill and produces anaverage correction signal which slowly adjusts the set point ofdownstream stand AGC system in a direction to correct the error.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of the drawingillustrates, in schematic form, a control system applied to a two standtandem rolling mill which utilizes and embodies the present invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT and 14 is exerted by backuprolls 16 and 17 and ascrewdown device 18 whose position, and hence theopening between the rolls, is adjusted by means of screw controls 19. Itwill be understood that the screw control 19 ofeach stand will act toadjust the roll opening between the working rolls l3 and 14 of eachstand in accordance with the magnitude of a reference signal supplied toinput lead 20. While screw controls have been shown, the roll openingsof the stands may also be controlled by hydraulic means responsive tothe reference signal.

The speeds of the motors driving the working rolls 13 and 14 of stands 1and 2 are controlled by speed regulators 21 which typically receivespeed signals on leads 22a and 22b from a process control computer (notshown) as is well understood by those skilled in the art. Also, it iscommon practice to provide means for regulating the tension of the stripbetween the stands to prevent looping or breaking of the strip. For thispurpose, there is shown a strip tension sensing device 23 bearingagainst the strip 10 between the stands 1 and 2 which provides a striptension feedback signal on lead 24 to a tension regulator 25 whichreceives a tension reference signal from the process control computer onlead 22c and provides a speed correction signal on lead 26 to the speedregulator 21 controlling the roll motor speed of upstream stand 1. Thecorrection signal adjusts the set point of the regulator in a properdirection to maintain the tension of strip 10 within operating limits.The mill control system thus far described is known and for that reasonhas been illustrated schematically and described in general terms.

According to the invention, the roll openings of the stands 1 and 2 arecontrolled in a novel manner to maintain the gage of strip 10 leavingthe stand 2 at some desired preset value. The manner in which thiscontrol system operates will now be described.

Each of the stands 1 and 2 is provided with a separate AGC system havinga feedback control responsive to a predicted exit gage from the rollbite of the associated stand. Considering first the control system ofstand 1 there is provided a gage measuring device such as an X-ray gage27 having an upper portion 27a producing X-rays and a lower portion 27bcomprising an X-ray sensor between which the strip 10 passes beforeentering the rool bite of stand 1. The X-ray gage 27 produces an entrygage signal G proportional to the thickness or gage of a measuredincrement of strip 10 as it approaches the roll bite of stand 1. Thegage may, for example, be arranged to make measurements at intervalscorresponding to the time required for one foot of strip to pass throughthe gage. These measurement signals are transmitted to a computercomponent 28 through lead 29, a buffer storage unit 30 and lead 31, thecomputer being used to predict, in a manner to be described, the exitgage of the strip 10 as it leaves the roll bite of stand 1.

The purpose of the buffer storage unit 30 is to delay the entry gagemeasurement signal for an interval corresponding to the time requiredfor the measured strip increment to pass from the X-ray gage 27 to theroll bite of stand 1. This avoids the time delay or transport lag effectin calculating the predicted exit gage used for control purposes. Toenable the buffer storage unit 30 to provide the desired time delayfunction automatically, it is provided with a timing signal on lead 32from a tachometer generator 33. The generator is driven by a roller 34which engages and is rotated by the strip 10 as it moves toward the rollbite of stand 1. Thus the ta chometer generator 33 produces a speedsignal V proportional to the strip speed as it approaches stand 1.Knowing the distance from the X-ray gage 27 to the roll bite of stand 1and the speed of the strip entering the stand, the time delay introducedin the gage measurement signal is determined by simple calculation ofthe buffer storage unit in a well known manner.

The computer unit 28 calculates the predicted exit gage of the strip 10leaving stand 1 by use of the constant volume flow rate principle asapplied to stand 1. According to this principle the volume flow rate ofthe strip into the stand must equal the volume flow rate of the stripleaving the stand. If the width of the strip is considered to remainconstant during the elongation caused by the working rolls of the standthen the product of the strip entry speed V and the strip entry gage Gmust equal the product of the strip exit speed V and the strip exit gateG Expressed mathematically:

G strip entry gage for stand l G strip exit gage for stand 1 V stripspeed entering stand 1 V strip speed leaving stand 1 1 Thus the exitgage G is given by the equation derived from (I):

The computer unit 28 calculates the predicted exit gage G according toequation (2) using three input quantities, namely, the strip entry gageG,,,, the strip entry velocity V, and the strip exit velocity V Thequantity G is supplied by the X-ray gage 27 through the buffer storageunit 30 to eliminate transport lag. The strip entry speed quantity issupplied by tachometer generator 33 through lead 35. The strip exitspeed quantity V is supplied via lead 35a by a second tachometergenerator 36 driven by a roller 37 which engages the strip between thestands 1 and 2.

The output signal G from the computer unit 28 is fed through lead 38 toa comparator unit 39 where it is compared with a desired exit gagesignal from a gage set unit 40 which is manually set by an operator. Anydifference between the predicted exit gage signal and the desired exitgage signal is fed as an error signal on lead 20 to the screw controlunit 19 of stand 1 to cause adjustment of the roll opening in a properdirection to reduce the error signal to zero. Thus it will be seen thatthe AGC system for stand 1 has a separate control loop into which afeedback signal is introduced which varies in accordance with thepredicted gage of the strip 10 as it leaves the roll bite of stand 1.

Considering now the AGC system for stand 2 there is provided anadditional computer unit 41 which calculates the predicted exit gage ofthe strip leaving stand 2 and generates a signal used as a feedback tocontrol the roll opening of stand 2. The computer unit 41 uses theconstant volume flow rate principle, explained above, as applied tostand 2 according to which:

G V G V where;

G strip entry gage for stand 2 G strip exit gage for stand 2 V stripspeed entering stand 2 V; strip speed leaving stand 2.

Thus the exit gage G is given by the equation derived from (3):

According to equation (4) computer unit 41 calculates the predicted exitgage G using three input quantities, namely, the strip entry gage G thestrip entry velocity V, and the strip exit velocity V The quantity G isderived from the quantity G calculated by the computer unit 28, thesetwo quantities being related since the exit speed of the strip leavingstand 1 is approximately equal to the entry speed of the strip intostand 2. However, to eliminate the transport lag effect the signal 0,,is routed to the computer unit 41 by leads 42 and 43 through anadditional buffer storage unit 44. This unit delays the entry gagesignal G for an interval corresponding to the time required for thestrip increment measured by X-ray gage 27 to pass from the roll bite ofstand 1 to the roll bite of stand 2 and in this manner eliminates thetransport lag which would otherwise occur. To enable the buffer storageunit 44 to provide the desired time delay function automatically itreceives a timing signal in lead 36a from the tachometer generator 36.The input quantity V is also provided by the tachometer generator 36 vialead 45 and the remaining input quantity V is provided by an additionaltachometer generator 46 via lead 47. To enable the tachometer generator46 to provide an output signal proportional to the quantity V it isdriven by a roller 48 which engages and is rotated by the stripdownstream with respect to the roll bite of stand 2.

The output signal G from computer unit 41 is fed to a comparator unit 49via lead 50 where it is compared with a desired exit gage signal from agage set 51 which is manually set by an operator. Any difference betweenthe predicted exit gage signal G from stand 2 and the desired exit gagesignal is fed as an error signal on lead 20 to the screw control unit 19of stand 2 to cause adjustment of the roll opening of stand 2 in aproper direction to reduce the error signal to zero. Thus, in a mannersimilar to the control of stand 1, the AGC system for stand 2 has aseparate control loop into which a feedback signal is introduced whichvaries in accordance with the predicted gage of strip as it leaves theroll bite of stand 2. In the case of the two stand tandem millillustrated this is also the predicted exit gage of the strip leavingthe mill.

For various reasons, the calculations of the predicted strip exit gagesfrom stands 1 and 2 by the computer units 39 and 41 may not always be100 percent accurate. For example, the diameters of the rolls 34, 37 and48 which drive the speed signal tachometers 33, 36 and 46 by not beaccurately known or they may change due to wear. To correct for smallsystem control errors of this nature there is provided a system formonitoring the actual measured gage of the strip leaving the mill andmaking any required corrections by a vernier type of adjustment to theroll opening control of stand 2. For this purpose there is provided asecond X-ray gage 52 having an upper portion 52a producing X-rays and alower portion 52b comprising an X-ray sensor between which the strip 10passes after leaving the roll bite of stand 2. The X-ray gage 52produces an exit gage signal proportional to the thickness or gage of ameasured increment of the strip as it leaves the roll bite of stand 2.These measurement signals are transmitted to a monitor unit 53 via lead54. By a known process of integration the monitor averages the exit gagethickness signal and transmits it to the comparator unit 49 via lead 55.6

lf there is any deviation between the actual exit gage and the desiredexit gage as determined by the setting of gage set unit 51, the setpoint of the screw control unit 19 of stand 2 is automatically adjustedin a direction to correct for the exit gage error. Because of theaveraging action of the monitor unit 53 the effect of any transport lagbetween the roll bite of stand 2 and the X-ray gage 52 is eliminated.Also, it should be understood that because of the relatively slow actionof the monitor system the time constant of the feedback signal on lead55 is long as compared with the time constant of the predicted feedbackgage signal on lead 50 so that it does not affect the stability of theAGC system controlling the roll opening of stand 2.

From the foregoing, it will now be apparent that there is provided, bythis invention, a control system for a tandem rolling mill thateliminates the effect of transport lag without direct coupling of theAGC systems controlling the roll openings of adjacent upstream anddownstream stands. This is accomplished by providing separate AGCsystems for each stand with each system utilizing as its primaryfeedback control signal the predicted gage of the strip leaving thestand. To implement this concept calculation of the downstream standexit gage utilizes, as an input, the calculated exit gage of theupstream stand with buffer storage delay to eliminate the effect oftransport lag caused by the time required for a measured strip incrementto pass between the roll bites of the adjacent stands. Because theadjacent stands are not directly coupled by the AGC controls the tensionregulator 25 can function in a normal manner to control interstand striptension without any instability caused by interaction between the striptension and gage controls.

While the invention has been illustrated as applied to a tandem rollingmill having two stands, the control principle is applicable to millshaving a greater number of stands. Thus, for example, if the mill hasthree stands there will be provided an additional AGC control system forstand 3 similar to that illustrated for stand 2 and the X-ray gage 52will be located downstream with respect to stand 3 with the gagecorrection signal on lead 55 applied to the screw control of stand 3rather than stand 2. Also, the system may be used with either analog ordigital type of control.

For clarity of illustration the computer units 28 and 41 and the bufferstorage units 30 and 44 have been shown as separate components. Inactual practice they can be combined with and form a part of the processcontrol computer which is programed to adjust the mill controlsautomatically.

While there has been shown what is presently considered to be apreferred embodiment of the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a control system for a tandem rolling mill com prising upstreamand downstream rolling stands, said stands having separate roll openingcontrols and being operative, successively, to reduce the thickness of astrip of material passing through the mill, the combination comprising:

means for producing strip speed signals V,, V, and V variable,respectively, in accordance with the speed of the strip entering theupstream stand, the speed of the strip between the upstream anddownstream stands, and the speed of the strip leaving the downstreamstand,

means for producing an entry gage signal G variable in accordance withthe entry gage of the strip entering the upstream stand,

first computer means receiving as input signals said strip speed signalsV, and V and said entry gage signal G for computing a predicted exitgage signal G for said upstream stand,

means for controlling the roll opening of the upstream stand inaccordance with said predicted exit gage signal G second computer meansreceiving as inputs said strip speed signals V and V and an entry gagesignal G for computing a predicted exit gage signal G for saiddownstream stand, said signal G being derived from said calculated exitgage signal G of said upstream stand, and

means for controlling the roll opening of the downstream stand inaccordance with said predicted exit gage signal G 2. The control systemof claim 1 wherein the calculation of the signal G is in accordance withthe equation ib lu I/ z and the calculation of the signal G is inaccordance with the equation 3. The control system of claim 1 includingdelay means for receiving the exit gage signal G and providing, as anoutput signal, the entry gage signal G which is proportional, butdelayed in time, with respect thereto, said delay means being controlledby the strip speed signal V to determine the amount of time delay.

4. The control system of claim 3 including means for measuring the gageof the strip delivered from the downstream stand and producing anaverage correction signal variable with deviation of the exit gage froma predetermined value and means controlled by said correction signal formodifying the control of the roll opening of the downstream stand tocorrect for measured errors in the gage of the strip delivered from thedownstream stand.

5. The control system of claim 4 including means for regulating thetension of the strip between the upstream and downstream stands.

6. The method of controlling the roll openings of adjacent upstream anddownstream rolling stands used to successively reduce the thickness ofstrip material passing through a tandem rolling mill, said methodcomprising:

controlling the roll opening of the upstream stand in accordance with apredicted upstream stand strip exit gage determined by multiplying anincrementally measured upstream stand strip entry gage by the ratio ofthe speeds of the strip entering and leaving this upstream stand; and,

controlling the roll opening of the downstream stand in accordance witha predicted downstream stand strip exit gage determined by multiplying,after a delay interval related to the transport time for the measuredincrement to pass between the upstream and the downstream stands, apredicted downstream stand strip entry gage by the ratio of the speedsof the strip entering and leaving the downstream stand, the predicteddownstream stand strip entry engage being derived from the predictedupstream stand strip exit gage.

UNITED STATES PATENT AND TRADEMARK OFFICE ETTFTQATE 0F CGREECTTN DPATENT NO. 3,782,153

DATED January 1, 1974 INVENTOR(S) Amos J. Winchester It is certifiedthat error appears in the above-identified patent and that said LettersPatent 3 are hereby corrected as shown below:

Column 1, line 26, cancel "unitl" and substitute until-.

line 52, cancel "sigal" and substitute -signal-.

Column 3, line 39, cancel "rool" and substitute roll.,

Q Column 4 line 21, equation (2) should read G =G (V /V line 65,equation (4) should read -G =G (V /V Column 5, line 48, cancel "by" andsubstitute may-.

Column 7, line 24 equation should read -G =G (V /V Q line 28, equationshould read G G (V /V Column 8, line 33, cancel "engage" and substitutegage-.

Signed and Scaled this twenty-seventh D ay Of April 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Office (mnmissimu'r uj'lurentsand Trademarks

1. In a control system for a tandem rolling mill comprising upstream anddownstream rolling stands, said stands having separate roll openingcontrols and being operative, successively, to reduce the thickness of astrip of material passing through the mill, the combination comprising:means for producing strip speed signals V1, V2 and V3 variable,respectively, in accordance with the speed of the strip entering theupstream stand, the speed of the strip between the upstream anddownstream stands, and the speed of the strip leaving the downstreamstand, means for producing an entry gage signal G1a variable inaccordance with the entry gage of the strip entering the upstream stand,first computer means receiving as input signals said strip speed signalsV1 and V2 and said entry gage signal G1a for computing a predicted exitgage signal G1b for said upstream stand, means for controlling the rollopening of the upstream stand in accordance with said predicted exitgage signal G1b, second computer means receiving as inputs said stripspeed signals V2 and V3 and an entry gage signal G2a for computing apredicted exit gage signal G2b for said downstream stand, said signalG2a being derived from said calculated exit gage signal G1b of saidupstream stand, and means for controlling the roll opening of thedownstream stand in accordance with said predicted exit gage signal G2b.2. However, to eliminate the transport lag effect the signal G1b isrouted to the computer unit 41 by leads 42 and 43 through an additionalbuffer storage unit
 44. This unit delays the entry gage signal G2a foran interval corresponding to the time required for the strip incrementmeasured by X-ray gage 27 to pass from the roll bite of stand 1 to theroll bite of stand 2 and in this manner eliminates the transport lagwhich would otherwise occur. To enable the buffer storage unit 44 toprovide the desired time delay function automatically it receives atiming signal in lead 36a from the tachometer generator
 36. The inputquantity V2 is also provided by the tachometer generator 36 via lead 45and the remaining input quantity V3 is provided by an additionaltachometer generator 46 via lead
 47. To enable the tachometer generator46 to provide an output signal proportional to the quantity V3 it isdriven by a roller 48 which engages and is rotated by the stripdownstream with respect to the roll bite of stand
 2. The output signalG2b from computer unit 41 is fed to a comparator unit 49 via lead 50where it is compared with a desired exit gage signal from a gage set 51which is manually set by an operator. Any difference between thepredicted exit gage signal G2b from stand 2 and the desired exit gagesignal is fed as an error signal on lead 20 to the screw control unit 19of stand 2 to cause adjustment of the roll opening of stand 2 in aproper direction to reduce the error signal to zero. Thus, in a mannersimilar to the control of stand 1, the AGC system for stand 2 has aseparate control loop into which a feedback signal is introduced whichvaries in accordance with the predicted gage of strip 10 as it leavesthe roll bite of stand
 2. In the case of the two stand tandem millillustrated this is also the predicted exit gage of the strip leavingthe mill. For various reasons, the calculations of the predicted stripexit gages from stands 1 and 2 by the computer units 39 and 41 may notalways be 100 percent accurate. For example, the diameters of the rolls34, 37 and 48 which drive the speed signal tachometers 33, 36 and 46 bynot be accurately known or they may change due to wear. To correct forsmall system control errors of this nature there is provided a systemfor monitoring the actual measured gage of the strip leaving the milland making any required corrections by a vernier type of adjustment tothe roll opening control of stand
 2. For this purpose there is provideda second X-ray gage 52 having an upper portion 52a producing X-rays anda lower portion 52b comprising an X-ray sensor between which the strip10 passes after leaving the roll bite of stand
 2. The X-ray gage 52produces an exit gage signal proportional to the thickness or gage of ameasured increment of the strip as it leaves the roll bite of stand 2.These measurement signals are transmitted to a monitor unit 53 via lead54. By a known process of integration the monitor averages the exit gagethickness signal and transmits it to the comparator unit 49 via lead 55.If there is any deviation between the actual exit gage and the desiredexit gage as determined by the setting of gage set unit 51, the setpoint of the screw control unit 19 of stand 2 is automatically adjustedin a direction to correct for the exit gage error. Because of theaveraging action of the monitor unit 53 the effect of any transport lagbetween the roll bite of stand 2 and the X-ray gage 52 is eliminated.Also, it should be understood that because of the relatively slow actionof the monitor system the time constant of the feedback signal on lead55 is long as compared with the time constant of the predicted feedbackgage signal on lead 50 so that it does not affect the stability of theAGC system controlling the roll opening of stand
 2. From the foregoing,it will now be apparent that there is provided, by this invention, acontrol system for a tandem rolling mill that eliminates the effect oftransport lag without direct coupling of the AGC systems controlling theroll openings of adjacent upstream and downstream stands. This isaccomplished by providing separate AGC systems for each stand with eachsystem utilizing as its primary feedback control signal the predictedgage of the strip leaving the stand. To implement this conceptcalculation of the downstream stand exit gage utilizes, as an input, thecalculated exit gage of the upstream stand with buffer storage delay toeliminate the effect of transport lag caused by the time required for ameasured strip increment to pass between the roll bites of the adjacentstands. Because the adjacent stands are not directly coupled by the AGCcontrols the tension regulator 25 can function in a normal manner tocontrol interstand strip tension without any instability caused byinteraction between the strip tension and gage controls. While theinvenTion has been illustrated as applied to a tandem rolling millhaving two stands, the control principle is applicable to mills having agreater number of stands. Thus, for example, if the mill has threestands there will be provided an additional AGC control system for stand3 similar to that illustrated for stand 2 and the X-ray gage 52 will belocated downstream with respect to stand 3 with the gage correctionsignal on lead 55 applied to the screw control of stand 3 rather thanstand
 2. Also, the system may be used with either analog or digital typeof control. For clarity of illustration the computer units 28 and 41 andthe buffer storage units 30 and 44 have been shown as separatecomponents. In actual practice they can be combined with and form a partof the process control computer which is programed to adjust the millcontrols automatically. While there has been shown what is presentlyconsidered to be a preferred embodiment of the invention, it will beapparent to those skilled in the art that various changes andmodifications may be made therein without departing from the spirit andscope of the invention. What is claimed as new and desired to be securedby Letters Patent of the United States is:
 2. The control system ofclaim 1 wherein the calculation of the signal G1b is in accordance withthe equation G1b G1a V1/V2 and the calculation of the signal G2b is inaccordance with the equation G2b Glb V2/V3
 3. The control system ofclaim 1 including delay means for receiving the exit gage signal G1b andproviding, as an output signal, the entry gage signal G2a which isproportional, but delayed in time, with respect thereto, said delaymeans being controlled by the strip speed signal V2 to determine theamount of time delay.
 4. The control system of claim 3 including meansfor measuring the gage of the strip delivered from the downstream standand producing an average correction signal variable with deviation ofthe exit gage from a predetermined value and means controlled by saidcorrection signal for modifying the control of the roll opening of thedownstream stand to correct for measured errors in the gage of the stripdelivered from the downstream stand.
 5. The control system of claim 4including means for regulating the tension of the strip between theuPstream and downstream stands.
 6. The method of controlling the rollopenings of adjacent upstream and downstream rolling stands used tosuccessively reduce the thickness of strip material passing through atandem rolling mill, said method comprising: controlling the rollopening of the upstream stand in accordance with a predicted upstreamstand strip exit gage determined by multiplying an incrementallymeasured upstream stand strip entry gage by the ratio of the speeds ofthe strip entering and leaving this upstream stand; and, controlling theroll opening of the downstream stand in accordance with a predicteddownstream stand strip exit gage determined by multiplying, after adelay interval related to the transport time for the measured incrementto pass between the upstream and the downstream stands, a predicteddownstream stand strip entry gage by the ratio of the speeds of thestrip entering and leaving the downstream stand, the predicteddownstream stand strip entry engage being derived from the predictedupstream stand strip exit gage.